The pet
table keeps track of which pets you
have. If you want to record other information about them, such
as events in their lives like visits to the vet or when
litters are born, you need another table. What should this
table look like? It needs to contain the following
information:
The pet name so that you know which animal each event pertains to.
A date so that you know when the event occurred.
A field to describe the event.
An event type field, if you want to be able to categorize events.
Given these considerations, the CREATE
TABLE
statement for the event
table might look like this:
Press CTRL+C to copymysql> CREATE TABLE event (name VARCHAR(20), date DATE, type VARCHAR(15), remark VARCHAR(255));
As with the pet
table, it is easiest to
load the initial records by creating a tab-delimited text file
containing the following information.
name | date | type | remark |
---|---|---|---|
Fluffy | 1995-05-15 | litter | 4 kittens, 3 female, 1 male |
Buffy | 1993-06-23 | litter | 5 puppies, 2 female, 3 male |
Buffy | 1994-06-19 | litter | 3 puppies, 3 female |
Chirpy | 1999-03-21 | vet | needed beak straightened |
Slim | 1997-08-03 | vet | broken rib |
Bowser | 1991-10-12 | kennel | |
Fang | 1991-10-12 | kennel | |
Fang | 1998-08-28 | birthday | Gave him a new chew toy |
Claws | 1998-03-17 | birthday | Gave him a new flea collar |
Whistler | 1998-12-09 | birthday | First birthday |
Load the records like this:
Press CTRL+C to copymysql> LOAD DATA LOCAL INFILE 'event.txt' INTO TABLE event;
Based on what you have learned from the queries that you have
run on the pet
table, you should be able to
perform retrievals on the records in the
event
table; the principles are the same.
But when is the event
table by itself
insufficient to answer questions you might ask?
Suppose that you want to find out the ages at which each pet
had its litters. We saw earlier how to calculate ages from two
dates. The litter date of the mother is in the
event
table, but to calculate her age on
that date you need her birth date, which is stored in the
pet
table. This means the query requires
both tables:
Press CTRL+C to copymysql> SELECT pet.name, TIMESTAMPDIFF(YEAR,birth,date) AS age, remark FROM pet INNER JOIN event ON pet.name = event.name WHERE event.type = 'litter'; +--------+------+-----------------------------+ | name | age | remark | +--------+------+-----------------------------+ | Fluffy | 2 | 4 kittens, 3 female, 1 male | | Buffy | 4 | 5 puppies, 2 female, 3 male | | Buffy | 5 | 3 puppies, 3 female | +--------+------+-----------------------------+
There are several things to note about this query:
The
FROM
clause joins two tables because the query needs to pull information from both of them.When combining (joining) information from multiple tables, you need to specify how records in one table can be matched to records in the other. This is easy because they both have a
name
column. The query uses anON
clause to match up records in the two tables based on thename
values.The query uses an
INNER JOIN
to combine the tables. AnINNER JOIN
permits rows from either table to appear in the result if and only if both tables meet the conditions specified in theON
clause. In this example, theON
clause specifies that thename
column in thepet
table must match thename
column in theevent
table. If a name appears in one table but not the other, the row does not appear in the result because the condition in theON
clause fails.Because the
name
column occurs in both tables, you must be specific about which table you mean when referring to the column. This is done by prepending the table name to the column name.
You need not have two different tables to perform a join.
Sometimes it is useful to join a table to itself, if you want
to compare records in a table to other records in that same
table. For example, to find breeding pairs among your pets,
you can join the pet
table with itself to
produce candidate pairs of live males and females of like
species:
Press CTRL+C to copymysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species FROM pet AS p1 INNER JOIN pet AS p2 ON p1.species = p2.species AND p1.sex = 'f' AND p1.death IS NULL AND p2.sex = 'm' AND p2.death IS NULL; +--------+------+-------+------+---------+ | name | sex | name | sex | species | +--------+------+-------+------+---------+ | Fluffy | f | Claws | m | cat | | Buffy | f | Fang | m | dog | +--------+------+-------+------+---------+
In this query, we specify aliases for the table name to refer to the columns and keep straight which instance of the table each column reference is associated with.